1. Field of the Invention
The present invention relates auditory fitting, and more particularly, to auditory fitting for multimodal stimulation device.
2. Related Art
Hearing loss is generally of two types, namely conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. In such cases, the hearing loss may often be compensated by the use of hearing instruments, which amplify a microphone signal. Hearing instruments deliver acoustic or mechanical energy to the ear. This may be through a column of air to the eardrum, or coupling of a transducer to the tympanic membrane, the skull, an ossicle, the round or oval window or any other structure suitable to lead the imposed mechanical energy to the hydro-mechanical system in the inner ear.
Sensorineural hearing loss, however, is due to the absence or destruction of the hair cells in the cochlea that are needed to transduce acoustic signals into auditory nerve impulses. Individuals suffering from a severe or profound form of this type of hearing loss are unable to derive benefit from these hearing instruments described above. This is because the natural mechanisms for transducing sound energy into auditory nerve impulses have been damaged. In such cases, hearing prosthetic implants, such as Cochlear™ and Nucleus™ implants produced by Cochlear Limited of Australia, have been developed to provide the sensation of hearing to such individuals. In this kind of hearing prosthetic implants, electrical stimulation is provided via stimulating electrodes positioned as close as possible to the nerve of the auditory nerve, essentially bypassing the hair cells in a normally functioning cochlea. The application of a stimulation pattern to the nerve endings causes impulses to be sent to the brain via the auditory nerve, resulting in the brain perceiving the impulses as sound.
The treatment of both of the noted types of hearing loss has been quite different, relying on two quite different principles to deliver sound signals to be perceived by the brain as sound. It is relatively common in hearing impaired individuals to experience severe sensorineural hearing loss for sounds in the high frequency range, and yet still be able to discern sounds in the middle to low frequency range, through the use of a hearing instrument, or naturally. Traditionally, in the majority of such cases, the recipient would only receive treatment to preserve and improve the hearing for the middle to low frequency sounds, most probably via a hearing instrument, and little would be done to attempt to restore the severe hearing loss for the high frequency sounds.
More recently, there has been an increased interest in hybrid device, such as Electro-Acoustical Stimulation (EAS), in which electrical stimulation of the cochlea is used in conjunction with acoustical stimulation. Such hybrid combination device may provide the recipient with the ability to derive benefit from both hearing instruments and prosthetic implants as described in above. The hearing instrument amplifies low frequencies of a sound signal while the hearing prosthesis electrically stimulates the middle and high frequencies of that sound signal. However, the transfer function of each stimulation component of such a hybrid system requires distinguished adjustment to be optimized for the recipient.